Method of and apparatus for highfrequency dielectric heating



Dec. 23,1947. e. F. RUSSELL METHOD OF AND APPARATUS FOR HIGH-FREQUENCYDIELECTRIC HEATING 3 Sheets-Sheet 1 Filed June 26, 1942 Dec. 23, 1947.s. F. RUSSELL METHOD OF AND APPARATUS FOR HIGH-FREQUENCY DIELECTRICHEATING Filed June 26, 1942 3 Sheets-Sheet 2 Dec. 23, 1947 RUSSELL I I2,433,067

METHOD OF AND APPARATUS FOB HIGH-FREQUENCY DIELECTRIC HEATING Filed June26,. 1942 s Sheets-Sheet s r l/4- WAVE-- PRESSLRL RULE AND CHI-HG Tl-ll2' Patented Dec. 23, 1947 METHOD OF AND APPARATUS FOR mon- FREQUENCYDIELECTRIC HEATING George F. Russell, Tacoma, Wash. Application June 26,1942, Serial No. 448,633 3 Claims. (01. 219-47) This application is acontinuation in part of my application, Serial No. 367,147, filedNovember 25. 1940 (now abandoned), fora method of forming plywood.

The process is preferably of the continuous type in which sheaves ofglue-spread veneers are passed between series of pressure rollers, heatbeing generated instantaneously in the glue lines to permit the desireduniting of the various layers.

Normally, plywood is formed b the batch method, that is, a number ofglued veneer sheaves are placed in powerful hydraulic presses and heatis applied by means of plattens which are in most instances heated bylive steam. Bonding agents are utilized whose bonding temperatures rangefrom about 100 F. to 390 F., a temperature just below the flash point ofthe wood being bonded. The heat from the plattens passes through thewood veneers and acts on the bonding agent at the glue lines. The heatis naturally less intense in the interior of the sheaf, that is, on theglue lines of the interior plies or a multiple ply sheaf, than in theoutside plies and a satisfactory bonding is not attained until the heatfrom the plattens has thoroughly penetrated the sheaf and raised thetemperatures of the inner glue lines substantially to that of the outerglue lines.

Diiiiculty has been experienced in the past due to the necessit forraising the temperature of the veneer close to its flash point toprovide quick penetration ofsuilicient heat at the interior glue linesfor satisfactory bonding; moreover, due to the wide temperaturevariations between the outer and inner plies, great difficulty has beenexperienced due to the tendency of the veneers to buckle, split andcheck.

It has been suggested to form plywood sheets in a continuous process bypassing glued veneers through a series of pressure rollers whileapplying heat by means of steam jackets and also by means of electricalresistance coils in the rollers themselves. So far a .is known none ofthese continuous processes utilizing heated rollers have gone intocommercial use due to the numerous difficulties attendant upon theiroperation, the

high loss of heat due to radiation from the rollers being one of themajor difficulties.

The present invention contemplates the formation of plywood by acontinuous process, as opp0sed to the old batch process, in which theplywood sheaf is slowly passed between a series of adjustable pressurerollers. The novelty of the process lies in the manner of producingbondin temperatures at the glue lines between the Veneers. The processcontemplates the utilization of high frequency radio waves oscillatingbetween suitable electrodes at opposite sides of the veneer sheaf toraise the temperature of the bonding agent sufficiently to permit of asuitable bond being formed between the sheets. It has been found throughexperimentation .that this method of heating the glue lines obviatesmany of the difiiculties attendant upon bonding methods heretofore inuse. lfhe old gradual heat penetration which existed in the hot plattenmethods has been eliminated and it is no longer necessary to passextremely high temperatures through the Wood veneers. The same desirableeffect is created at the inner glue lines as is created at the outerglue lines and thus the long standing problem of unbalanced heattransfer is eliminated. The high frequency wavesoscillating between theelectrodes penetrate and result in action on all glue lines in almostidentical fashion, except where the'sheaf is very thick in which casethe center glue lines become the hottest.

In bonding veneer layers by the use of radio frequency waves, the wavespass through the veneer layers and have their efiect on the glue linesbetween the layers. The resistance'of the glue to the passage of thewaves through the sheaf of veneer and glue causes a rapid increase inthe temperature of the glue at the'glue lines. The waves oscillate at arate per second equal to the frequency at which the output circuit ofthe oscillator is tuned between electrodes representing two or moreelements of the output circuit. These electrodes are placed in proximityto each other so that the passage of the radio waves alternating betweenthem will be as concentrated as possible. The veneer sheaves passbetween electrodes of the output circuit of the oscillator oroscillators and the glue is acted upon as a result of the Waves passingbetween the electrodes.

The electrodes may be plates, bars, rollers, or metal strips. Ithas beenfound that upper and lower pressure rollers bearing directly on theveneer will act as electrodes when they are so isolated as to allow theoutput circuit to be matched with the oscillator frequency or viceversa. If the output circuit of an oscillator is tuned with two or moreelectrodes as output load elements of the circuit, standing waves arepresent in fixed shape or in" fixed relation to the outputcircuit,-based on the tuning andfrequency thereof. For example, if anoscillators output circuit load consists of two bar orroller-electrodes,

certain points on the electrodes will be surrounded by a greater fieldintensity than other points. This is evidenced by the existence of orlack of current at such points. When two electrodes are placed closetogether lengthwise and areeach fed from the same end, the opposite orfree ends possess a greater current field than the ends from which theyare fed by the pickup system of the circuit.

The invention may be used for drying green veneers and similar materialsas well as for creating heat in and bonding glue in plywood. In dryingveneers the physical problem involved is not complicated. It is firstnecessary only to raise the wood and its water content to a temperaturesufficient to cause total orpartial evaporation of the water content.

Natural veneer, as it comes from the green log, varies in moisturecontent from 25 to-40%. The present method of drying is to subject theveneers to steam heat at temperatures of from 320 to 370 F, for from to11 minutes. The green veneer is usually carried between rollers thelength of long dry kilns. Heat penetrates wood from the outside to theinside. The time cycle involved depends upon the amount ofdehumidiflcation desired.

Heat caused by penetration of high frequency radio waves is distributedthrough the wood with the greatest heat being created at the center ofthe material. This type of heating from the center out tends to drivethe moisture out toward the surfaces where it may evaporate into theair. The reverse is true in the case of steam heating or other types ofexternal heating. Heat applied externally to wood tends to overheat andcook the outer surfaces, injuring the fibres, in order to force enoughheat to the center to produce drying at that point.

It is possible and feasible to eliminate the moisture from wood Or othermaterial exclusively by the use of internal heat induced by radio waves.It is, however, less expensive to start the dryin with radio waves, 1.e., bringing the mass up to a given temperature and then evaporating themoisture content by means of the cheaper steam heat.

The present method has been found to be particularly efllcaceous in thedrying of thick stock,

which normally takes hours or even days to dry by the usual steammethod.

In plywood manufacture many types of glues are used. Those employed inwater-resistant plywoods differ from those used in non-waterproofplywoods. The glues used in the waterproof type difier from thoseused inthe water-resistant type.

The-exterior grade of whose glue lines are waterproof, are manufacturedby the hot plate method, the bond being set with temperatures on theglue lines of above 212 F. This method of manufacture requires runningthe steam in the plattens to high temperatures, above 212 F. andfrequently as high as 370 F., in order that the glue lines when set willwithstand the boiling water tests without delamination.

The water-resistant grade of plywoods, i. e., those whose glue lineswill resist water, but which are not waterproof, are usually made on hotplate presses with platten temperatures of from 180 to 240 F. Thetemperature penetrating to the center glue lines rarely exceeds 160-180F. and will vary widely below 160 F., depending on the type of glue, thetime cycle of heating, thickness and number of plies and other factors.

The non-waterproof and non-water-resistant plywoods, i. e., those 4grades, 1, e., cold pressed, are formed without application ofartificial heat. It requires from 2-8 hours to set a batch of coldpressed plywood at normal room temperatures.

In applying heat to the glue hues we are concerned mainly with thewaterproof and waterresistant grades of plywood. Thi type of manufactureinvolves the setting of those types of bonds wherein heat on the gluelines is necessary to create sohdification of the thermo-setting typesof glues and the liquefaction of the thermo-plastic types. These twotypes of bonding materials will be further explained later in thisspecification.

It.is therefore an object of this invention to evolve a continuousprocess for the formation of plywood from wood or other veneers in whicha relatively uniform heating effect is produced on all of the glue linesbetween the veneers.

Another object of the invention is the provision of means whereby theoscillation of high frequency radio waves may be utilized to afford arelatively uniform heating effect at all glue lines throughout theveneer sheaf.

A further object or the invention is to reduce the temperatures andpressures previously necessary to provide a satisfactory bond betweenveneers and thereby to reduce the cost and waste attendant on priormethods of forming plywood.

Another object is to provide an improved method and apparatus for dryinggreen veneers and similar materials.

Other and further objects of the invention will become apparent as thisspecification proceeds.

Referring to the drawings,

Fig. 1 is an elevation of one embodiment of a machine designed for usein the present method, showing pairs of driven pressure rollers.alternately connected at opposite ends in the output circuits of aseries of oscillators and acting as electrodes;

Figs. 1a and 1b illustrate one method of mounting and insulating thedriven pressure rollers or electrodes to isolate the electrodes fromadjacent materials;

Fig. 2 shows one method of attaching electrodes E in the output circuitof the oscillator;

Fig. 3 is an alternative hook-up in which electrodes E are connected inthe output circuit at their centers;

Fig. 4 shows an embodiment in which electrodes E are connected atopposite ends in the output circuit;

Fig. 5 is a section view showing in detail the electrical fieldssurrounding electrodes of the roller type;

Fig. 6 is an elevation of an embodiment of the invention in which setsoi electrodes E are positioned adJacent to rollers R and sep ratetherefrom;

Fig. 7 is a side elevation of the apparatus covered in Fig. 6, showingthe field set up by electrodes E;

Fig. 8 is an elevation of another embodiment of the invention similar tothat illustrated in Fig. 6 and in which the electrodes E comprise thinstrips of metal whose extremity contacts the plywood sheaf;

Fig. 9 is a side elevation of the device illustrated in Fig. 8, showingthe electrical fields sur-- rounding this type of electrode;

Fig. 10 is a side elevation of a set of flexible electrodes as per Fig.8;

Fig. 11 is a side elevation of a set of stiff metal electrodes as perFig. 8;

Figs. 12a to '12). inclusive, show various typ of feeds between theoutput circuit of the os- Fig. 14 is a pictograph showing a fullstanding voltage wave in its entirety, with nodes at X, F

and K.

Fig. 15 is a further pictograph illustrating the quarter wave heatingeffect; the full standing voltage wave on the electrodes broken downinto its four component quarter waves.

Fig. 16 is a plan view of an apparatus for use with thermo-plasticbonding materials in which the plywood sheaf is first passed throughheating electrode rollers and then through a series of pressure andcooling rollers which set the bond; and

Fig. 17 is a side elevation of the apparatus shown in Fig. 16. 1

In the drawings, Figs. 1-5, E indicates suitable driven pressurerollers. These rollers may be constructed of any suitable material,preferably of metal, and are preferably cylindrical in shape. They areso mounted in a suitable frame I as to allow for vertical adjustmentwith respect to lower rollers E. The pairs of rollers may be spaced atany desired interval. Any suitable means for forcing rollers E towardrollers E, such as mechanical tension, hydraulic pressure, etc., may beutilized.

In Fig. 1a, for example, rollers E are shown suitably mounted on slidingframe 2 as by standoff insulators 3. Frame 8 is suitably longitudinallyslotted to allow vertical movement of sliding frame 2. frame i as bymeans of wing nuts 4. Rollers E are shown suitably mounted on standofiinsulators. By varying the distance between the upper and lower rollersthe pressure exerted by these rollers may be varied as desired. 5

In Fig. 1 a plywood sheaf P is shown suitably disposd between the setsof upper and lower rollers. As illustrated the plywood sheaf consists ofsix veneer layers with five glue lines therebetween. Suitable caul orbacking sheets may be applied, if desired, on either side of the sheafprior to insertion between the rollers to distribute the pressure moreevenly. Oscillator coils O are'illustrated. Rollers E and E are shownconnected as electrodes in the output circuits of oscillators O in suchfashion that oscillating high frequency waves generated in oscillatorcoils 0 will 6 on rollers E and E' is such that 132 is the poin whereexists the greatest electrostatic field intensity and therefore heatwill be concentrated in the sheaf passing between the electrodes. Atpoints Bl on the rollers there will be less heat due to lower fieldintensity between these points. At point A, the center point of thepick-up coil, there will be the least possibility. of heat, as point Ais the electrical center of the circuit, 1. e.. the

current antinode or voltage node under no-load condition. These waveforms will be further explained in the portion of the specificationrelating to Figs. 14 and 15. The standing wave on the electrodes isevidencedby the varying amountsof voltage and current at differentpoints in the circuit. The electrostatic field intensity between thelectrodes is greatest at'BI, the free end of illustrated in Fig.1.

Sliding frame 2 is adjustable in the electrodes. It will therefore beseen that the greatest heating effect on the glue lines of 3. plywoodsheaf will be exerted at and about the free end 132 of the electrodes.Thus, if each set of electrodes was fed from the same side there wouldexist at the opposite side a muchmore intense-electrostatic-field than.at the feeding side.

This would cause an intense heating of th glue lines at one side of thesheaf and a lower heating of the glue lines at the other side of thesheaf and would result in an uneven lateral bonding of the plywood. Toavoid such a result it is proposed that the sets of electrode rollers beied alternately'from opposite sides- Thishookupis As illustrated; thefirstset of electrode rollers E and E are fed from their right-handextremities'by oscillator coils O, creating an. electrostatic field ofgreatest intensity about their left extremities and thus about'the leftportion of the plywood sheaf P. The next set of electrode rollers arefed from oscillator 0' from their left extremities, creating thegreatest electrostatic field intensity about their right extremities andthus about the right portion of ply- -wood sheaf P. As the rollerelectrodes rotate slowly, passing theplywood sheaf in the directionindicated by the arrow;v itv will be seen that first the left-hand sideof the sheaf is subjected to an electrostaticIfield of great intensityand that subsequently the right-hand side of the sheaf is subjected tosuch a field. This alternate .subjection of the opposite sides of thesheaf to intenseelectrostatic fields causes uniform activity on the gluelines on each side of the sheaf and tends to produce a perfectlybalancedbond therein.

alternate between rollers E and E so as to pass through that portion ofthe plywood sheaf lying between and djacent to them. The pairs ofpressure rollers may be driven by any suitable means.

1 and worm gears 8. Worm gears 8 are separated electrically from eachset of the rollers by insulated couplings. desired speed to pass theveneer sheaf through the press at whatever speed may be requiredv to Asillustrated in Fig. 1a, roller shafts 5 carry gears 6 which are drivenfrom drive shaft In. Fig.3 there is illustrated an alternative hookup in"which rollers E and, E are fed from theircentral points Bl, instead offrom their extremities; The pick-up coil of oscillator O is They may berotated at any I illustrated connectedto the electrodes at centralpoints Bl.

The greatest electrostatic field intensity will therefore exist. aboutterminal points B2 at the extremitiesof electrodes Eand E.

' InFig; 4 anotherhookup is illustrated. This hookup is another method.of, equalizingfield ini tensity on the; electrodes. As illustrated, thepick-up coil of oscillator O'is connected to opposite, ends ofelectrodes E and E. Thus, as greatestfield intensity exists about pointsB2 on'each It will be noted that rollers E and electrode and least,intensity about points Bi the. field intensity about theelectrodes willbe "equalized. The hookup shown in Fig.2 is more satisfactory than'thatof Figs. 3 and 4'since by its use it is possible mechanically toequalize differences in potential inthe' field on the rollers by addingrollers fed in the opposite direction.

v:notpossible.withithehookupsoiffil igs 3. V

7 and 4. Both Figs. 3 and 4, however, have their own distinct advantagesand are to be preferred as compared to the Fig. 2 method for many usesother than that illustrated in Fig. 1.

Fig. 5 shows in detail the electrostatic field between electrodes E andE, and illustrates that field intensity is greatest directly between theelectrodes.

Fig. 6 shows another embodiment of the invention in which electrodes Eare placed adjacent driven pressure rollers R and separate therefrom inpairs above and below the sheaf. Electrodes E are preferably cylindricalin shape and are preferably formed of metal or of any other suitableconducting material. They are preferably of much smaller diameter thanpressure rollers R. The reduced diameter and the lessening electricalcapacity of these smaller electrodes make it possible to use lower wavelengths and therefore higher frequencies on the operating oscillators,resulting in greatly increased heating effect.

Electrodes E may be in the form of small rollers; they may be stationarytubes or bars or in any other suitable form. They preferably beardirectly on the material bein passed between them to provide the mostefiective application of the high frequency field existing between them,but need not touch the material if such a condition is desirable, due toextremely wet material passing between them which might be apt to shortout the field.

It will be noted that pairs of electrodes E are fed alternately fromopposite ends to balance the heating effect on the material P, thegreatest field intensity being about points B2 at the extremity of theelectrodes E.

Fig. 7 is a side elevation of the apparatus shown in Fig. 6, showing thenature of the fields set up about pairs of electrodes E. It will benoted that the greatest field intensity occurs directly between theelectrodes, the intensity decreasing away from the electrodes.

Fig, 8 is a plan view of another embodiment of the invention, similar tothat shown in Fig. 6, but utilizing a diiferent type of electrode. Inthis embodiment, as in the embodiment shown in Fig. 6, the smallerelectrical capacity of pairs of electrodes E allows the use of shorterwave lengths and therefore higher frequencies of the operatingoscillators feeding the electrodes. Electrodes E, in this embodiment,preferably consist of thin strips of stiff or flexible metal suspendedin the output circuits of the oscillators in such position that theirextremities K bear directly on material P passing between electrodes E.Portion K of this type of electrode is its contact foot. It will benoted, from Figs. 10 and 11, that contact feet K of electrodes E arefarther apart when material is passing between them than when nomaterial is present. This arrangement is designed to insure a firmcontact at all times between the electrodes E and the material beingheated, resulting in an extremely even and efficient heating effect.

As shown in Figs. 10 and 11, curved spring metal electrodes may be used,which are fixed at points H or, as in Fig. 11, stiff strips of metal maybe used, which are pivotally mounted at points H to allow passage ofmaterial P.

It will be noted that in this embodiment also, the sets of electrodes Eare fed alternately from opposite sides from oscillator coils O, toequalize the heating effect in the plywood sheaf. Here again, the areaof greatest field intensity is about points B2, at the extremities ofelectrodes E.

Fig. 9 is a side elevation of the apparatus shown in Fig. 8, showing theelectrical fields surrounding electrodes E. It will be noted that thepoint of maximum field intensity lies directly between contact feet K ofthe electrodes E.

Figs. 10 and 11 illustrate two types of strip electrodes. In Fig. 10 theelectrodes E comprise flexible metal strips fixedly mounted at points Hin such fashion as to be forced to position K by the passage of thesheaf between them. In Fig. 11 electrodes E comprise stiff metal stripswhich are not curved and which are mounted to oscillate about points Hto enable the passage of the sheaf therebetween. It will be noted thatboth types of strip electrodes illustrated maintain a constant contactwith the surfaces of the sheaf at all times, but may be fixed so as notto make direct contact under circumstances, one of which was describedabove.

Figs. 12a to 12 inclusive, illustrate some of the various types of feedswhich may be utilized between the output circuits of the oscillators andthe electrodes E.

The particular feed utilized depends upon the nature of the materialbeing processed, the type of processin done, the type of field desiredand the type of radio frequency generator used.

Fig. 12a shows a straight feed to the electrodes without any variablecapacity either in the output tank or the pick-up circuit, connectingelectrodes E,

Fig. 12b shows variable capacity in the pick-up circuit and none in theoutput tank circuit.

Fig. 12c shows a variable capacity in the output tank circuit and nonein the pick-up circuit.

Fig. 12d shows variable capacity in both circuits.

Fig. 12c shows rounding one electrode and one side of the pick-upcircuit, while the other side of the pick-up circuit has both variablecapacity and the only line connection.

Fig. 12f illustrates variable capacity between two elements of thepick-up circuit.

Fig. 13 is a side view showing the types of fields set up between thevarious types of electrodes, 1. e., roller, small bar, and metal strip.The figure illustrates that field intensity is greatest perpendicular toline X-X' between the center of elecoutside surfaces are subject toatmospheric radiation of heat, while the interior is not subject toradiation. Other factors also contributing to the nature of the heatingeffect are the thickness of the material being processed, the density ofthis material, the distance of the electrodes apart, etc.

Fig. 14 is a pictograph showing a standing voltage wave in its entirety,with voltage nodes at X, F and X. The current curve of this wave willlag. in a push-pull, self-excited oscillator of half wave rectifier typeby degrees behind this voltage curve. The points of zero voltage orpotential( voltage nodes), therefore, will be the same on line X-X' asthe points of maximum current, or current'anti-nodes. The pictographillustrates that a standing quarter wave eliminates superfluous currentanti-nodes between X and D. Such points are wasteful of energy.

As was stated in the descriptions of Figs. 12a

to f, inclusive, the method of feeding electrodes E, regardless ofwhether they are roller, bar or strip electrodes, as per Figs. 1, 6 or'7, has a substantial bearing on the type of wave standing on theelectrodes. In the case of the roller electrodes it is desirable to feedas much energy as possible into the rollers to get the maximum effect onthe material passing between them. The consequent elimination of currentantinodes in the circuit is most highly desirable as superfluous currentantinodes dissipate heat in the circuit. Any heat discharged in theelectrical factors is wasted as far as the heating effect on theprocessed material is concerned.

On a full standing current wave, there will be two currentantinodestherein. These are the points at which the current wave form is farthestaway from the zero line X-X'. It is, therefore, most highlydesirable inthe present instance to establish a circuit which has a. full standingwave on the entire output circuit instead ofa quarter wave or halfwave.-

In Figs. 2, 6, and 8 it will be noted that the center point of thepick-up coil connected to each end orthe electrodes, is marked A. A isshown" in Fig. 14 as a line drawn perpendicular to base line X-X' andpassing through the center of the pick-up circuit as shown above thewave form. The extremities of the electrode, B2, opposite the center'ofthe pick-up coil, are shown as B2 in the pictograph, resting on thecrest of the wave form at a point directly above D on base line X-X'.Point Bl, on the electrodes is shown to-be at the extremity of theelectrode where it is fed from the pick-up coil. The total pick-upcircuit, as described, will be split into two parts, namely from A to B2on the upper electrode and from A to B2 on the lower electrode. It willbe seen, therefore, from the pictograph, that since XD on line X-X' isone quarter wave, the point of greatest voltage will be the crest of thevoltage wave form shown in Fig. 14, directly above D, or the voltageantinode. I

Fig. 15 is a further pictograph of the voltage wave form as shown inFig.14, butdealing with the wave from points X to D of Fig, 14,increasing its size, and superimposing onthis section of the wave formboth electrodesE in the output circuit of the oscillator, and completingthe cycleof the full standing wave from B2 back to the zero line X asshown in Fig. 14 from D to F on line X-eX';

It will be noted that point A is'halfway between X.X' in Fig. 15 and isthe. v oltagenode and the current .antinode of the half wave voltageform shown. Points B2 on the curve of the wave-formfi are the currentnodes and voltage antinodes of this half'waveform- Center line X-X' isthe same electrical depiction .of line X,-*-X. asshown pressure andcooling.

10 greatest at the current antinode, or voltage node, where current lagsvoltage by 90 degrees, it will be'seen that at Bl there will be lesselectrical effect on the material being processed than at B2 as Bliscloser to A than is B2 and the form of the wave is a pure sine curve.

Fig. 16 is a plan view of an apparatus designed specifically for usewith the thermo-plastic types of bonding agents, as distinguished fromthe socalled thermo-setting bonding agents. It will be seen that theapparatusdisclosed in Figs. 16 and 1'? comprises a combination of pairsof heating roller electrodes E, and pressure rollers R, so arranged thatthe sheaf first passes through electrode rollers E, where thethermo-plastic bonding agent between the plies is raised toliquification temperature. The sheaf then passes through sets ofpressure rollers R, where the bond is set by Electrode rollers E arepreferably cylindrical in shape, of 'any'suitable conducting material,and are of any desired size. They may be driven from any suitable sourceby worm gearing or other means and they are suitably insulated as bystandoff insulators or other means from the frame of the machine. Asillustrated, electrodes E are preferably arranged in pairs and arepreferably alternately fed from opposite sides from oscillator coils 0so as to equalize heating effect. Any desired number of electroderollers E may be utilized, depending upon the thickness, density andnature of the plies being processed.

Pressure rollers R. are shown. They are preferably arranged in pairs atany desired interval, and are preferably cylindrical in shape and of anysuitable material and size. erably so mounted that the distance betweenrollers may be adjusted to regulate the pressure exerted. They may,forexample, be mounted as mi e. 1.

Pressure. rollers R may be driven from any suitable source, as by woringears, etc. As illustrated, cooling pipes C are providedconnecting theupperand lower pressure rollers. ;Cooling 1 pipesjC' carry a suitablecooling medium,-- i; e., water, brine. or other refrigerant and aresoiarranged as tocooi pressure'rollers R'-internally. As illustrated,the. cold refrigerant is introduced into pressure rollers R at the endof the apparatus opposite to theheating exit. Thus the rollersnearestthe heating electrodes will. carry the warmest refrigerantandthose at the end will ".carrythe coldest. This arrangement tends tosupplyia gradual cooling ,to the thermoplastic bonding agent asitis-subjected to pressure; resulting in a perfect bond; The coolingagent,

was it leaves the warm end outletycan be recooled in Fig. 14, halfwaybetween'the-jelectrodes E51 It I will,. therefore, be seen that'thereiisgsaouarter.

standing wave on that portion of the electrical;

' jcircuit containing the upper electrode and 'there is another quarterstanding'wave'on that portion of the electrical circuit containing theopposite electrode.

The line X-B3 represents the distance between .quarter standing wavefrom A to the end of the electrode E and another'quarter wave from thatpoint to the center line, or a half wave from A til-X;

. Since the intensity and'penetrating effect are in a suitablerefrigerating plant and-then recirculated.

The roller electrodes E and the pressure rollers R may be driven at anydesired speed, depending union the nature of the 'ply'sheaf, itsthickness. and the bonding agent used. The speed is also regulateddepending upon the moisture content of the veneers, the bonding point ofthe thermoplastic used, and other factors.

. If desired,,an endless belt may be provided over A the rollers E andR, or cauls may be utilized above and below the sheaf.

The heating section of the apparatus illustrated in Figs. 16. and 1'7,namely that composed of electrodes E above and below the materialpassing there-between may be supplemented, in another application by ahydraulic flat bed press, the plattens. of which-are cooled either bywater or They are pref brine or some other cooling media, andsubstituted in the process illustrated above for the pressing rollers Rand the cooling coils C illustrated as the second part of the apparatusshown in Figs. 16 and 17, respectively. In this method of operation, thepackage being heated through the section comprising electrodes E wouldpass from the effect of the radio frequency fields and be pressed by astationary hydraulic platten press, the heat being withdrawn from saidpackage and the bond being set therein by the action of simultaneouspressing and cooling in the said hydraulic press. This method asdifferentiated from that illustrated graphically in Figs. 16 and 17 willprevent any possibility of warpage of the veneer, strain, checking,cracking or other such finishing difiiculty.

Nothing in this specification should limit, therefore, the applicationof this method of using a thermo-plastic bonding agent to strictly themethods shown in Figs. 16 and 17. The method equally applies to thebatch press method, when it is used in conjunction with the continuousheating section above described, namely the first section of Figs. 16and 17.

So far as thermal setting resin bonds are concerned the hydraulic pressmay be used without a cooling agent when the cycle is so timed as toallow the bond to set when the package being treated has been runthrough the heating section composed of electrodes E and subsequentlyplaced for pressing between hydraulic press plattens.

The apparatuses illustrated in Figs. 1, 6 and 8 are designed for bondingveneers with the thermo-setting types of bonding agents. The phenols,ureas, seed meal and other formaldehydes, blood albumen and Bakeliteglues and others comprise the thermo-setting agents. Thermosettingsynthetic resins are the ones most commonly used as a base for plywoodglues. They exhibit the characteristic of setting or becoming hard on arise in temperature. A thermal setting resin having a critical settingtemperature of 160 F. will set at about that temperature when treatedfor a given length of time. Once set they will not reliquify on theapplication of additional heat.

It is necessary, when bonding plywood with thermo-setting materials, toapply heat and pressure simultaneously. The heat is necessary to set theglue, the pressure is necessary to keep the fibres of alternate layersin intimate contact to make a satisfactory wood bond when the glue setsup.

The thermo-setting agents are difficult to control in a high frequencyfield as they create a very low resistance path between the electrodeswhen an excess is used, as the excess squeezes out in a blob frombetween the layers of veneer and contacts the electrodes. The blobcarbonizes on contact and forms a flash or are between the electrodes.This are reflects back into the oscillator, varying the frequency andunbalancing the circuit.

In working with thermo-stting compounds the pressure must be applied atthe right time. If the bonding agent solid fies without enough pressure,an open or porous bond results and the glue line and bond will beunsatisfactory. Critical setting temperature and speed of passage of thesheaf are of primary importance when working with these agents.

The thermo-plastics. on the other hand, exhibit the property of settingon cool ng, and will reliquify on the application of sufficient he t f rsetting. No thermo-plastics are being used commercially at present inplywood manufacture as no equipment is at present available which isadaptable to such use. I have found, however, that with a machine of thetype shown in Fig. 21 the use of thermo-plastics as bonding agents isfeasible. I have used one of the thermo-plastics with marked success ona machine of this type. The thermo-plastic used is a syntheticcommercial product made with vinyl as a base. I have found that it ispossible to create any type of plywood desired by varying the criticalmelting temperature of the thermo-plastic. For example, a thermo-plasticwhose critical liquefying temperature is above 212 F. will withstand allwater tests and is waterproof both as to soaking and boiling. Athermo-plastic whose critical temperature is 160 F. will withstandsoaking without delamination but will not withstand boiling temperature,namely, 212 F. This bond will be water-resistant at temperatures belowits critical temperature.

The problem of arcing, as discussed in connection with thermo-settingbonding agents, 1. e., the ureas, formal-dehydes, etc., is non-existentwhen using some thermoplastic bonding agents in an apparatus similar tothat shown in Fig. 16.-

The new process is preferably carried out as follows: A sheaf of eithergreen or dry veneers is assembled with a suitable bonding agent betweeneach veneer. It has been determined that either wet or substantially drybonding agents may be utilized, as the heat created at the glue lines issufficiently intense to raise either type to binding temperature. Thehead of the assembled sheaf is then inserted between the first pair ofpressure rollcr electrodes and the rollers are set in motion. Thepressure exerted by the rollers may be regulated gradually to increasefrom the entering end of the machine to the terminal end thereof, or aconstant pressure may be exerted. Satisfac ory results have beenobtained with each type of adjustment. The temperatures generated at theglue lines'may vary from about F. to about 390 F. depending upon thethickness of the sheaf and the binding agent used. The pressure exertedby the rollers may vary from a few pounds per square inch to manyhundreds of pounds per square inch depending upon the nature of thesheaf and the degree of bond therein desired, or whether the wood isnatural or resin impregnated.

Greater activity in the glue lines is created by radio waves of afrequency of twenty million cycles per second than by radio waves of anorder of two million cycles per second. Successful operations by radiofrequency penetration have been conducted on frequencies of an order offifty million cycles per second and as low as one million cycles persecond. The field intensity between electrodes in the output circuit ofan oscillator operating with an input power of 1,500 watts and tuned toresonance at a high frequency of the order of fifty million cycles persecond is such as rapidly to raise the temperature of the gluescurrently used in hot plate methods to well beyond the bonding point.The point to which the temperature is raised may be regulated by varyingthe intensity of the field, the power of the radio frequency generator.tuning the circuit ofl of peak resonance. varying the speed of passageof the veneers through the area of radio frequency activity and byvarying the factors contributing to the bonding temperature of the glue.The flash point of fir veneer is something over 390 F. and

. 13 the increase in the heat of the glue line should be kept below thistemperature so as not to injure the wood fibre.

With the present process it is possible to utilize green, undriedveneers and a substantially dry binding agent and procure excellentresults. Due to the extremely high temperatures generated in the gluelines and in the wood by the oscillations of the high frequency waves itis possible to create a satisfactory bond while using pressures farbelow that of 150 pounds per square inch normally required in themanufacture of plywood.

Satisfactory results can be obtained by numerous alternativeapplications of this method. For example, the glue-spread veneers may bepassed between slowly driven roller electrodes until the glue linetemperature is raised to the binding point. The plies may then be placedin a hydraulic press and subjected to pressure until the bond iscomplete. As an alternative method of pressing the sheaf it may bepassed through a series of pressure rollers which complete the bond.

Another method is to place the glue-spread I veneers between flat plateelectrodes which are then pressed together and move on a continuoustrack until the glue lines have been heated to the binding point and thebond completed.

, In any of these methods the frequency of the oscillations between theelectrodes may be varied so that the temperatures at the glue-lines willbuildup rapidly or slowly. The bond may be set at peak temperature or ona declining temperature, depending upon the veneer used, the glue, andthe frequency of the wave oscillations between the electrodes.

The advantages of this new method of heating and bonding plywood arenumerous. For the first time. a simultaneous effect on all of the gluelines, both interior and exterior is produced. No difiiculty has beenexperienced with buckling or which comprises passing such materialthrough a heating zone and subjecting it during its passage therethroughto a plurality of successive high frequency fields of force establishedbetween successive pairs of electrodes extending transversely acrOss thepath of the material, andfeeding the electrodes of each pair ofelectrodes from the same end and successive pairs of electrodes fromopposite ends to more evenlyv balance the heating effect on saidmaterial throughout said zone.

2. An apparatus for heating dielectric materials comprising successivepairs of electrodes between which the material to be heated passes andproviding a heating zone comprising the successive fields of forcebetween said electrodes, and high frequency oscillator means for feedingsaid electrodes, the electrodes of each pair extending transverselyacross the path of said material and being fed from the same end andsuccessive pairs of electrodes being fed from opposite ends for moreevenly balancing the total heat distribution in the material throughoutsaid zone.

3. An apparatus as claimed in claim 2 wherein said oscillator meanscomprises a plurality of high frequency oscillators, each pair ofelectrodes being connected to the output of one of said oscillators toestablish a high frequency field of force through warping of the veneersand waste is reduced to a minimum. It is possible to produce uniformstock continually and the savings in labor and processing are great.Lastly, due to the action of the high frequency waves at the glue lines.and due to the high permeation of the binding agents it is possible tocreate high grade plywood with a small fraction of the glue formerlyused.

The invention is susceptible of many embodiments and adaptations.Nothing in this specification is intended to limit its scope. Attentionis directed to the appended claims for this purpose.

What is claimed is:

1. The method of heating dielectric materials the material.

' GEORGE F. RUSSELL.

REFERENCES CITED The following references are of record in the fileofthis patent:

UNITED STATES PATENTS Number Name Y Date 2,153,131 Bohme Apr. 4, 19391,945,546 Hansell Feb. 6, 1934 2,017,093 Hansell Oct. 15, 1935 1,978,021Hollmann Oct. 23, 1934 352,131 Johnson Nov. 9, 1886 2,179,261 KellerNov. 7, 1939 1,706,675 Osgood Mar. 26, 1929 2,204,617 Peel et al June18, 1940 2,331.723 Percival 'Oct. 12, 1943 2,109,323 Smith Feb. 22, 19382,113,714 Stein Apr. 12, 1938 2,231,457 Stephen Feb. 11, 1941 2,291,807Hart, Jr. Aug. 4, 1942 2,288,269 Crandell June 30, 1942 1,900,573MacArthur Mar. 7, 1933 2,226,871 Nicholas Dec. 31, 1940

